RCL1 (RNA terminal phosphate cyclase like 1) is a nucleolar protein essential for ribosomal small subunit (40S) biogenesis. As a core component of the SSU processome, RCL1 functions in early pre-rRNA processing at sites A0, A1, and A2, promoting proper maturation of 18S rRNA 1. RCL1 activates the GTPase BMS1 by facilitating GDP/GTP exchange, though notably lacks cyclase activity despite its nomenclature. Critically, RCL1's stability and nucleolar localization depend entirely on its interaction with BMS1; without this binding, RCL1 undergoes ubiquitin-proteasome degradation 1. Beyond ribosome biogenesis, RCL1 has emerged as clinically significant in neuropsychiatric disease. A rare missense variant (rs115482041) segregates with depression across multiple family generations, explaining 2.9% of genetic variance and showing significant association in independent populations 2. RCL1 demonstrates striking co-localization with specialized astrocytes in primate brains, suggesting neurodevelopmental relevance 2. Additionally, RCL1 copy number variations associate with diverse neuropsychiatric phenotypes including catatonia, hallucinations, and cognitive decline 3. Non-coding variants in RCL1 influence neutrophil GM-CSF signaling in inflammatory bowel disease 4, and RCL1 is prioritized among 24 genes implicated in chromosome 9 deletion syndrome affecting multiple organ systems 5.